1. Field of the Invention
Exemplary aspects of the present invention generally relate to an image forming apparatus, such as a copier, a facsimile machine, or a printer, and more particularly, to a developing unit that performs continuous printing and an image forming apparatus including the developing unit.
2. Description of the Background Art
Related-art image forming apparatuses, such as copiers, facsimile machines, printers, or multifunction printers having at least one of copying, printing, scanning, and facsimile functions, typically form an image on a recording medium according to image data. Thus, for example, a charger uniformly charges a surface of an image bearing member; an optical writer projects a light beam onto the charged surface of the image bearing member to form an electrostatic latent image on the image bearing member according to the image data; a developing device supplies toner to the electrostatic latent image formed on the image bearing member to make the electrostatic latent image visible as a toner image; the toner image is directly transferred from the image bearing member onto a recording medium or is indirectly transferred from the image bearing member onto a recording medium via an intermediate transfer member; a cleaning device then cleans the surface of the image carrier after the toner image is transferred from the image carrier onto the recording medium; finally, a fixing device applies heat and pressure to the recording medium bearing the unfixed toner image to fix the unfixed toner image on the recording medium, thus forming the image on the recording medium.
When outputting a large number of recording media sheets for an extended period of time or image forming devices such as a fixing device and a developing unit are in operation for an extended period of time, the internal temperature of the image forming apparatus and the temperature of parts employed in the image forming apparatuses rise undesirably.
In order to prevent overheating of the image forming apparatus, generally, a cooling device such as a fan and a duct are employed to adjust the internal temperature. In particular, a high-speed image forming apparatus employs an air conditioner to adjust the internal temperature effectively.
When outputting small-size recording media sheets continuously, the temperature of a fixing roller employed in the fixing device where the recording media sheets do not come into contact rises locally. To address this difficulty, there is known an image forming apparatus that prevents irregular temperature of the fixing roller by monitoring the temperature of the fixing roller and extending temporarily intervals between the previous and the subsequent recording media sheets.
An amount of reduction in the internal temperature of the image forming apparatus by the known fan and the duct is limited due to the size and the internal configuration of the image forming apparatus, and arrangement of parts in the image forming apparatus. Furthermore, the temperature of the developing unit cannot be monitored directly so that the temperature of developer and sliding parts such as a shaft bearing in the developing unit increases significantly, melting undesirably toner in the developer.
In view of the above, a known approach includes reducing the temperature of a developing motor of the developing unit by calculating a change in the temperature of the developing motor and perform image forming processing intermittently when an estimated temperature of the developing motor reaches 100° C. or more. The image forming processing is performed intermittently until the temperature of the motor drops to 80° C. or less.
In this approach, the rise in the temperature is suppressed by calculating and estimating the temperature of the developing motor. However, the toner still melts in the developing unit.
In another approach, in a case in which a large amount of toner is consumed such as when a number of writing dots are equal to or greater than a threshold value, the image forming operation is performed intermittently to prevent toner from sticking to a toner regulation member (for example, a developing sleeve) that regulates a toner layer on the developing roller. Accordingly, the temperature of the toner (the toner layer) on the developing roller is reduced.
However, in a case in which the number of writing dots is small, the temperature of the developer and the sliding member such as the shaft bearing of the developing unit increases significantly. When this happens, the toner melts undesirably in the developing unit as well.